linux-kernel - RE: Missing clobber on alternative use on Linux UM 32-bit

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Date:   Sun, 5 Nov 2023 15:47:19 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nadav Amit' <nadav.amit@...il.com>,
        Anton Ivanov <anton.ivanov@...-begemot.co.uk>
CC:     Richard Weinberger <richard@....at>,
        "linux-um@...ts.infradead.org" <linux-um@...ts.infradead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: RE: Missing clobber on alternative use on Linux UM 32-bit

From: Nadav Amit
> Sent: 04 November 2023 09:41
> 
> > On Nov 4, 2023, at 11:34 AM, Anton Ivanov <anton.ivanov@...-begemot.co.uk> wrote:
> >
> > On 04/11/2023 09:25, Nadav Amit wrote:
> >>
> >> I was reading (again) the x86 C macro of “alternative()” and I was a bit
> >> surprised it does clobber the flags (“cc”) as a precaution.
> >>
> >>  #define alternative(oldinstr, newinstr, ft_flags) \
> >> 	asm_inline volatile (ALTERNATIVE(oldinstr, newinstr, ft_flags) : : : "memory")
> >>
> >> Actually there seems to be only one instance of problematic cases - in um/32-bit:
> >>
> >>  #define mb() alternative("lock; addl $0,0(%%esp)", "mfence", X86_FEATURE_XMM2)
> >>  #define rmb() alternative("lock; addl $0,0(%%esp)", "lfence", X86_FEATURE_XMM2)
> >>  #define wmb() alternative("lock; addl $0,0(%%esp)", "sfence", X86_FEATURE_XMM)
> >>
> >> Presumably, if XMM or XMM2 are not supported, there would be instances where addl
> >> would be able to change eflags arithmetic flags without the compiler being aware
> >> of it.
> >>
> >> As it only affects 32-bit Linux UM - I don’t easily have an environment to test
> >> the fix. An alternative (word-pun unintended) is to add “cc” as a precaution
> >> to the alternative macro.
> >>
> > Application alternatives in um is presently a NOP. It always uses the "blunt and heavy instrument" -
> the most conservative option.
> >
> > It is on the TODO list.
> 
> Thanks for the quick response. But I don’t see how it prevents the problem
> (it actually makes it worse - affecting XMM/XMM2 CPUs as well) since you
> keep the “lock; addl $0,0(%%esp)” in the running code, affecting eflags
> without telling the compiler that you do so through a “cc” clobber.

gcc always assumes that inline asm changes "cc" - there is no need
to add a 'clobber' for it.

	David

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